Treatment of oil wells



Patented July 8, 1941 TREATMENT OF OIL WELLS Sterling P. Hart, Long Beach, Calif., assignor to The Texas Company, New York, N. Y., a cor.-

poration of Delaware No Drawing. Application July 21, 1938, Serial No. 220,507

12 Claims.

This invention relates to the drilling of wells, and more particularly to the control of the heaving shale encountered in the drilling of such wells.

More specifically my invention relates to a method of drilling wells into and through heaving shale without the said shale heaving, and involves preventing the access to the shale of any substantial amounts of water by building up on the walls of the well bore a filter cake which is substantially impermeable to the passage of water. The formation of this waterproof filter cake is accomplished by first form.- ing a filter cake containing dispersed casein and then thereafter hardening the casein by subjecting it to the action of a casein-hardening agent.

Drilling muds are almost universally used in the drilling of wells employed for tapping underground collections of oil, gas, brines and water. These muds fulfill various functions, the most important of which are to assist in the removal of cuttings from the wells, to lubricate the drill tools and the drill pipes which carry-the tools, and to assist in the sealing of formations of gas, oil and water which may be encountered at various levels by the formation on the walls of the hole of a filter cake consisting of the solid constituents of the drilling mud.

In the course of drilling wells, the bores may encounter certain geological formations which are termed heaving shale formations or more simply heaving shale. The term denotes shale strata which do not remain consolidated during the drilling operations. In some cases the shale heaves or caves into the hole when the drilling tools are removed therefrom, and in other cases the shale may heave in the course of the actual drilling. In the latter case the shale may bind or pinch the drill stem, thereby preventing its rotation or even resulting in its destruction.

There are several types of heaving shales, each of which react difierently, inasmuch as some may be controlled merely by mechanical methods, whereas others require chemical treatment for successful control. Various theories have been advanced for explaining the heaving f shales which may be controlled by mechanical means. One is that the heaving of shale is caused by pockets of high pressure gases which exist in the crevices and partings of the shale formations.

Another theory is that this class of heaving shale is under considerable unrelieved stress and that the drilling of a hole through the shale forms an opening into which the shale can slide or cave to partially relieve the inner stress.

The class of heaving shales which require chemical treatment for successful control apparently disintegrate when they are contacted with an aqueous drilling mud, and'it is apparently the water content of the drilling mud which brings about this disintegration.

According to my invention, shale formations which normally disintegrate upon contact with water may be penetrated successfully in drilling operations without heaving by forming a substantially water impermeable filter cake on the walls of the hole, which cake prevents the access of any substantial amounts of water to the shale. I have found that a special technique is required in forming the water impermeable filter cake, and it is this technique which is the basis of my invention.

I have observed that heaving shales of the type which require chemical treatment normally do not disintegrate immediately upon penetration by the drilling tools, and that the heaving shales may be penetrated a limited distance before disintegration occurs. On the basis of this information I employ a drilling mud containing a small amount of dispersed casein which reduces considerably the permeability to water of the filter cake on the walls of the hole. I employ this drilling mud for a limited time, for example over the period required to drillfrom five to one hundred feet of heaving shale, and then substitute for the casein-contaiing drilling mud a drilling mud containing a small propor-v tion of a hardener for casein. In this way there is formed on the face of the heaving shale a filter cake which is substantially impermeable to the passage of water, even when the hydrostatic pressure is of the order of 5,000 pounds per square inch or more.

Although I am unable to advance a theory explaining the success of the method of treatment which I have discovered, I believe that what occurs when employing the casein and hardening agent is that whenthe first filter cake is laid down by the drilling mud containing dispersed casein, the casein occupies the interstitial spaces between the particles of clay or other finely divided solids which composethe solid phase of the drilling mud, and thereafter when the drilling mud containing the casein-' hardening agent is employed some of the aqueous phase of this drilling mud penetrates into the first filter cake, and by hardening the casein in situ brings about the formation of a substantially impervious filter cake.

The casein-containing drilling muds may be prepared in a wide variety of ways, and I do not intend to limit myself tojany particular method of prepartion or to any particular composition,

and in practicing my invention I intend the use,

of any drilling fluids containing aqueous dispersions of casein.

Casein is a member of the group of proteins and exhibits colloidal properties and is amphoteric in that it can act either as an acid or a base. It does not dissolve in water, but upon soaking therein it rapidly swells up. Dispersion of the casein is most conveniently effected by treating swelled casein with a water solution of an alkaline material such as borax, ammonia, an alkali metal bicarbonate, an alkali metal carbonate, lime, sodium metasilicate, sodium silicate, or any equivalent alkaline materials. It is advantageous to heat the solution of the alkaline material prior to its addition to the swelled casein, inasmuch as the dispersion of the casein is more rapid and the resulting product is more uniform. Where the casein dispersion is to be stored for any appreciable length of time it may be desirable to incorporate therein small proportions of preservatives such as phenols, cresol, camphor, pine oil and the like retarding the growth of bacteria or mold.

In order that those skilled in the art may understand thoroughly the preparation of dispersed casein, I give herewith several examples which may be used in the practice of my invention:

Example 1 Soak 100 pounds of casein in 31 gallons of water for at least 15 minutes. Then add a solu tion of 18 pounds of potassium carbonate dissolved in 12 gallons of water, the solution being at a temperature in the neighborhood of 110 F. Mix thoroughly and then add 29 gallons of water.

Example 2 Soak 100 pounds of casein in 30 gallons of water for at least 15 minutes, then add to the swelled casein a hot solution of borax prepared by dissolving 15 pounds of borax in 3 gallons of water. Thereafter add 36 gallons of water and stir thoroughly.

Example 3 Soak 100 pounds of casein in 27 gallons of water and then add thereto after having permittedthe casein to soak for at least 15 minutes a suspension of fresh slaked lime formed by mixing 25 pounds of slaked lime with 12 gallons of water and stirring thoroughly. When the mixture is uniform add '70 pounds of 40 B. sodium silicate and mix thoroughly. Then incorporate in the mixture 9. solution of 2 /2 pounds of copper chloride in about 2 gallons of water.

The amount of alkaline material to be used in preparing the alkaline dispersions of casein is dependent upon the activity of the particular alkaline material selected for the purpose and it is therefore difficult to state specifically the quantities to use. However, I have found that in most cases the quantity of alkaline material is preferably in the range of from 10 to 25% by weight of the dry casein.

Inasmuch as I prefer the use of dispersions containing from 5 to 20% by weight of dispersed casein the foregoing three examples show the preparation of dilute forms of casein dispersions. However, it is to be understood that'more concentrated dispersions may be prepared particularly where it is necessary to transport the casein dispersions appreciable distances.

In preparing. the casein-containing drilling mud the dispersed casein is added to the drilling mud, usually in an amount of from 10% to 25% by volume. Upon adding the casein dispersion to the drilling mud a temporary and slight increase in viscosity is usually observed, but the viscosity usually drops after the mud and the casein dispersion have been thoroughly mixed. The casein dispersion is compatible with most forms of drilling muds, including not only dispersions of natural clays in water, but also dispersions of weighting materials such as iron oxide, barytes, dispersions of bentonites, and in fact dispersions of any finely divided solids.

The drilling mud which is employed for waterproofing the casein-containing filter cake is prepared by mixing with a drilling mud a small proportion of a chemical compound which has the property of hardening casein. There are a Wide variety of chemicals which have this property, including among others organic aldehydes such as acetaldehyde, formaldehyde, furfuraldehyde, inorganic salts such as potassium dichromate, ferric chloride, and certain organic acids including tannic acid and the like. Of these materials I prefer to employ formaldehyde in the form of the commercial solution inasmuch as it is extremely active and has proved very successful.

.In employing the hardening agents only a small quantity is necessary, usually only about 2% by volume of the volume of casein dispersion being employed. In the case of the inorganic salts, these are employed in the form of their saturated water solutions and usually in the amount of from 2 to 5% by volume of the volume of the casein dispersion used.

My invention may with advantage be applied in the following way. For example, the caseincontaining drilling mud is employed while drilling from 5 to feet or more of heaving shale whereby the walls of the bore hole are lined with a filter cake containing dispersed casein. Drilling is then interrupted and a drilling mud containing a casein-hardening agent is introduced into the well in an amount sufiicient to more than fill that part of the hole lined with the new filter cake. This drilling mud is then slightly agitated as by raising and lowering the drilling tools or by raising the drilling tools and rotating them back to the bottom of the hole. Finally, after the mud has been sufficiently agitated, the drilling tools are withdrawn well above the area being treated and the mud allowed to remain quiescent for a period of from one-half to twelve hours or more to permit the casein-hardening agent to react with the dispersing casein. At the end of this time the drilling mud is displaced and drilling with the casein-containing drilling mud is resumed.

In practicing my invention I prefer to separate the casein-containing drilling mud from the drilling mud containing the casein-hardening agent in order to avoid any possibility of contamination of the casein-containing drilling mud, even though experiments have demonstrated that mix ing the two muds results in little change in the physical properties of the mixture. This separation of the two muds is most effectively accomplished by interposing a small quantity or slug of untreated mud between them.

As a typical example of the application of my invention I give herewith a description of the method followed for successfully drilling through heaving shale encountered in a well.

The drilling mud was prepared from natural clays and water, and to this drilling mud was added approximately 20% by volume of the casein dispersion disclosed in the foregoing Example No. 2. The casein-containing drilling mud formed in this way was used in the regular mannen'being circulated down the drill pipe to the locus of drilling and then upwardly to the surface in the space between the face of the formation and the drill pipe.

In drilling this well I employed the caseincontaining drilling mud for the time necessary to continuously drill through from about 50 to about 100 feet of heaving shale. When this thickness of heaving shale had been penetrated the caseincontaining filter cake laid down on the walls of the heaving shale was treated with formaldehyde by substituting for the casein-containing drilling mud a drilling mud containing 2% by volume of 40% formaldehyde. In order to prevent the deterioration of the casein-containing drilling mud by the formaldehyde-containing drilling mud, the two muds were separated in circulation by a small slug of ordinary drilling mud.

The amount of formaldehyde-containing drilling mud used was slightly more than sufficient to fill that portion of the well bore which was lined with the fresh filter cake. I effected agitation of the drilling mud by adding the formaldehyde--containing drilling mud with the drilling tool at the bottom of the hole, then when the formaldehyde-containing drilling mud was in place, rotating the drilling tools slowly. The tools were then slowly raised to a point slightly above the area being treated after which they were rotated downward without circulation until the bit was again on bottom. By following this procedure several times I found it possible to assure the correct amount of agitation. After this the drilling bit was raised above the area undergoing treatment and the reaction allowed to proceed for a period of about twelve hours before drilling ahead into the heaving shale.

After the formaldehyde-containing drilling mud, had been displaced drilling was continued with the regular casein-containing drilling mud. By using this technique I successfully drilled to the depth of almost 6,000 it, most of the well bore traversing very easily disintegrable heaving shale.

Obviously many modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof, and only such limitations should be imposed as are indicated in the appended claims.

I claim:

1. In the drilling of wells with the use of a drilling mud, the method of preventing the heaving of shale encountered in the well which comprises first circulating through the well a drilling mud containing dispersed casein and then introducing into the, well a drilling mud containing an agent adapted to harden the casein.

2. In the drilling of wells with the use of a drilling mud, the method of preventing the heaving of'shale encountered in the well which comprises first circulating through the well a drilling mud containing dispersed casein to form a filter cake and then introducing into the well a drilling mud containing an agent adapted to harden the casein, the, casein and casein-hardening agent being present in an amount suiiicient to yield a water-impermeable filter cake.

3. In the drilling of wells with the use of a drilling mud, the method of preventing the heav ing of shale encountered in the well which comprises first circulating through the well a drilling mud containing an aqueous alkaline dispersion;

of casein and then introducing into the well a drilling mud containing a casein-hardening agent.

4. In the drilling of wells with the use of a drilling mud, the method of preventing the heaving of shale encountered in the well which comprises circulating through the well a drilling mud comprising water, finely divided solids, and an aqueous dispersion of casein and so constituted as to form a filter cake on the walls of the well, discontinuing the circulation of this mud, and then introducing into the well a drilling mud comprising water, finely divided solids, and a hardening agent for the dispersed casein.

5. In the drilling of wells with the use of a drilling mud, the method of preventing the heaving of shale encountered in the well which comprises circulating through the well a drilling mud comprising Water, finely divided solids, and an alkaline dispersion of casein, and so constituted as to form a filter cake on the walls of the well, interrupting the circulation of this drilling mud by the introduction into the well of a drilling mud containing a casein-hardening agent, this drilling mud being adapted to render impermeable to water the filter cake laid down by the first drilling mud.

6. In the drilling of wells with the use of a drilling mud, the method of preventing the heaving of shale encountered in the well which com prises drilling a limited distance through the heaving shale while employing a drilling mud containing dispersed casein and adapted to yield a casein-containing filter cake, then substituting therefore a drilling mud containing a caseinhardening agent, agitating this mud slightly and then allowing it to remain quiescent for the time necessary to render the casein-containing filter cake impermeable to water.

7. In the drilling of wells with the use of a. drilling mud, the method of preventing the heaving of shale encountered in the well which comprises drilling a limited distance through the heaving shale while employing a drilling mud comprising a finely divided solid, water, an alkaline reagent and dispersed casein and adapted to yield a casein-containing filter cake, stopping drilling and the circulation of the caseimcontain ing drilling mud, introducing into the well a small portion of untreated drilling mud followed by a drilling mud containing about 2% by volume of a casein-hardening agent, the said mud being used in an amount suificient to more than fill the portion of the hole bounded by the-fresh filter cake, agitating this drilling mud slightly and then allowing it to remain quiescent for a period of from one-half to twelve hours, after which the drilling mud is displaced and drilling resumed with the casein-containing drilling mud.

8. In the drilling of wells with the use of a drilling mud, the method'of preventing the heaving of shale encountered in the Well which comprises drilling through from 5 to feet of heaving shale while employing a drilling mud comprising a finely divided solid, water, an alkaline reagent and dispersed casein, and adapted to form a casein-containing filter cake, stopping drilling operations and after introducing a slug of untreated drilling mud into the drill pipe, fill ing the portion of the-hole bounded by the fresh filter cake with a drilling mud containing about 2% of a 40% formaldehyde solution, agitating this drilling mud for a short period of time by raising and lowering the drill tool and then permitting the drilling mud to remain quiescent for a period of from one-half to twelve hours.

9. In the drilling of wells with the use of a drilling mud, the method of preventing the heaving of shale encountered in the well which comprises drilling through from 5 to 100 feet of heaving shale while employing a drilling mud.- comprising a finely divided solid, water, and from to 25% by volume of a dispersion of casein in a hora-x solution, the dispersion containing from 5 to by weight on a dry basis of casein, the drilling mud being adapted to yield acaseincontaining filter cake, then stopping drilling operations and after introducing a slug of untreated drilling mud into the drill pipe, filling the portion of, the hole bounded by the fresh filter cake with a drilling mud containing about 2% of a 40% formaldehyde solution, agitating this drilling mud for a short period of time by raising and lowering the drill tool, then permitting the drilling mud to remain quiescent for a period of from one-half to twelve hours and finally removing the drilling mud and resuming drilling operations with the casein-containing drilling mud.

10. In the method of preventing the heaving of shale in the drilling of a well through shale which tends to heave, the steps of circulating through the well a drilling mud containing dispersed. casein to form a casein containing mud sheath for the bore hole of the well, and then circulating through the well a fluid containing a constituent which hardens the casein and forms a water impermeable mud sheath therewith.

11. In the drilling of a well through shale which tends to heave, the method of preventing the heaving of shale which comprises drilling a certain distance through the shale with a drilling mud containing dispersed casein to produce a water-proof coating for the bore hole containing the dispersed casein, then discontinuing drilling and introducing into the well a different fluid containing a casein hardening reagent, allowing the said different fluid 'to stand for a period of time in the well in contact with the bore hole throughout the said certain distance drilled to form a water-proof coating thereon, then removing the said different fluid and continuing drilling with the first mentioned drilling mud.

12. The method of forming a water-proof coating on the bore hole of a well which comprises first introducing into the 'Well and into contact with the wall of the bore hole thereof a fluid containing dispersed casein, and then introducing into said well and into contact with said wall of the bore hole a fluid-containing a casein hardening reagent.

STERLING P. HART. 

